The Captain was an ordinary (slightly stupid) human from Brooklyn. One night, while on a bender in the streets of Brooklyn, two aliens of unknown origin named Spa-fon and Squa-tront, came across the man who was to become the Captain. They granted him the "Messianic Sidda-Complex," the "Heartstar of the space between galaxies." He was tasked with using his newfound power and glory to "make this nightmarish land of 'Brooklyn' a better place." He then used his newly-gained superhuman powers to beat up the aliens because he "thought when you hit leprechauns on the head they turned into gold coins." Finding that this was untrue, he bent over with the intent of robbing them, and instead vomited onto his extraterrestrial benefactors.
The Captain possesses what, are by his description, a "generic set of special superhero abilities" as a result of possessing the Heartstar.
Superhuman Strength: The Captain is so absurdly strong that he once smacked someone's spine out through their lungs just by patting them on the back; in his defense, he was quite extraordinarily drunk. At his peak, he can lift approximately 95 tons.
Superhuman Stamina: The Captain's augmented musculature generates less fatigue toxins during physical activity than the musculature of a normal human. While the full limits of his stamina aren't known, he can at least exert himself at peak capacity for several hours without tiring.
Superhuman Durability: The Heartstar has augmented the Captain's bodily tissues, making them much more resistant to physical injury than the tissues of a normal human. He can withstand high caliber bullets, falls from great heights, powerful energy blasts, and great impact forces without sustaining injury.
Superhuman Sight: The Captain's sight has been augmented by the Heartstar, allowing him to see much further and with greater clarity than an ordinary human.
Flight: The Captain is able to propel himself through the air for extended periods of time. The maxumum speed he can reach while in flight isn't exactly known, but it is known to be significantly lower than the Speed of Sound.